^simply stated.
You can leave ur lights on longer or even get higher voltage. Believe it or not htey sell algae 'growing kits'... they are little pill looking things that you place in your tank and it somehow increases ur algae. Also, add lettuce to ur tank, that's a tall tale but its said to add algae too. THis may help ur growth but will also help feed ur fish a healthy diet. I wouldn't go drastic and change things like redcorsair suggested (redcorsair is absolutely right but if you go that far u may never be able to come back)... the dark side is known to be uncontrollable, something you are attempting to do.